The only thing that doesnt make them very suitable for boost applications is that they have an internal fuel pressure regulator (with internal I mean that it sits in the white assembly, not in the pump casing), and that fuel pressure regulator is not vacuum (or boost) regulated but a set value.
For the 3.0si/z4m thats maybe less of an issue as they run on a fairly high pressure (I estimate 5 bar or so) but the 3.0i pump is regulated at 3,5bar (as far as capacity goes its the same internal pump I believe), so if you have 0,5 bar boost, that leaves 3.0 bar fuelpressure to get proper atomazation in/from your injectors.
With boost and thus the large pressure difference between vacuum and full boost I think boost/vacuum dependant regulated pressure would be better with the proper tuning (different fuel pressure automatically means new tuning of course).
This is btw the fuelpump on the m54 (pre-fl) versions:
Can length is 82mm, overall lenght (without the barbs/connector) is 97mm.
Can thickness is 39/46mm
No fancy qr code yet on this version, too old
(these are the only markings)
I'm not sure if its equivalent to the walbro 400 or 450. The 450 is e85 compatible and the 400 is the pump gas version of basically the same pump. I have no idea if a stock z4 is e85 compatible (we dont get that here afaik).
These bmw pumps are probably specifically made for bmw with non generic/bmw only electric connectors molded into the housing.